Optical modulator and optical module

ABSTRACT

An optical modulator includes an optical modulator chip configured to optically modulate an optical signal using an electrical signal input thereto; and a relay substrate configured to relay and couple the electrical signal to the optical modulator chip. The optical modulator chip includes a signal electrode and a ground electrode for the electrical signal, formed along a waveguide for the optical signal. One end of the optical modulator chip is arranged to face the relay substrate. An electrode connection portion coupling the electrical signal to the relay substrate by wire is provided at the one end. A distance between a tip of one end of the signal electrode in the electrode connection portion and the end of the optical modulator chip is less than a distance between a tip of an end of the ground electrode in the electrode connection portion and the end of the optical modulator chip.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2016-151501, filed on Aug. 1,2016, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

Embodiments of the invention relate to an optical modulator and anoptical module.

2. Description of the Related Art

Recently in the optical communication field, to cope with rapidlyincreasing traffic, the shift to higher bands such 100 G or 400 G[bits/sec] has advanced and devices used therein such as an opticalmodulator are also demanded to support the higher bands. Among widebandoptical modulators are those in which LiNbO₃, InP, or Si is used in thesubstrate (a chip). Of these, optical modulators in which LiNbO₃(hereinafter, referred to as “LN”) is used in the chip are used as amain component in respects of insertion loss, transmissioncharacteristics, and the controllability.

According to a conventional technique, for example, improvement of theripple of the high-pass characteristics is facilitated by forming afirst gap between a first ground electrode and a signal electrode andforming a second gap between a second ground electrode and the signalelectrode (for example, refer to WO 2005/111703). According to anotherconventional technology, the thickness of an end portion of a signalelectrode is made thinner than the thickness of an electrode of aninteraction portion (for example, refer to U.S. Patent ApplicationPublication No. 2008/0193074).

SUMMARY OF THE INVENTION

According to one aspect of the present invention, an optical modulatorincludes an optical modulator chip configured to optically modulate anoptical signal using an electrical signal input thereto; and a relaysubstrate configured to relay and couple the electrical signal to theoptical modulator chip. The optical modulator chip includes a signalelectrode and a ground electrode for the electrical signal, formed alonga waveguide for the optical signal. One end of the optical modulatorchip is arranged to face the relay substrate, and an electrodeconnection portion configured to couple the electrical signal to therelay substrate using a wire is provided at the one end. A distancebetween a tip of one end of the signal electrode positioned in theelectrode connection portion and the end of the optical modulator chipis less than a distance between a tip of an end of the ground electrodepositioned in the electrode connection portion and the end of theoptical modulator chip.

Objects, features, and advantages of the present invention arespecifically set forth in or will become apparent from the followingdetailed description of the invention when read in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan diagram of an example of a configuration of an existingoptical modulator;

FIG. 2 is an enlarged view of an electrode connection portion of anoptical modulator chip and a relay substrate depicted in FIG. 1;

FIGS. 3A and 3B are explanatory plan diagrams of chippings generated inthe existing optical modulator chip;

FIGS. 4A and 4B are plan diagrams of an optical modulator chip accordingto a first embodiment;

FIG. 5 is an enlarged view of FIG. 4A, depicting a state of the wireconnection;

FIGS. 6A, 6B, 6C, 6D, 6E, 6F are cross-sectional diagrams of an exampleof fabrication steps of the optical modulator chip according to thefirst embodiment;

FIG. 7A is a chart of characteristics of the optical modulator chipaccording to the first embodiment;

FIGS. 7B and 7C are diagrams of examples of LN chips;

FIGS. 8A and 8B are plan diagrams of an optical modulator chip accordingto a second embodiment;

FIGS. 9A and 9B are plan diagrams of an optical modulator chip accordingto a third embodiment;

FIGS. 10A and 10B are plan diagrams of an optical modulator chipaccording to a fourth embodiment; and

FIGS. 11A, 11B, and 11C are plan diagrams of an optical modulator chipaccording to a fifth embodiment.

DETAILED DESCRIPTION OF THE INVENTION

A configuration of an existing optical modulator chip, an opticalmodulator, and an optical module will be described. In the presentinvention, a LN chip will be described as an example of the opticalmodulator chip.

FIG. 1 is a plan diagram of an example of the configuration of anexisting optical modulator. The optical modulator 100 includes, inside ahousing (a package) 101, an LN chip (an optical modulator chip) 102including a substrate material that is a ferroelectric such as, forexample, LN, a relay substrate 103 that relays and couples to the LNchip 102, an electrical signal input thereto from outside the package101, parts 104 such as a terminating resistor and a bias T provided onthe LN chip 102 portion in the terminating portion of the electricalsignal, an optical fiber 105 for a light beam to enter or exit the LNchip 102, and the like. The optical modulator 100 is provided as atransmitting unit of the optical module.

The optical module includes a laser diode (LD), the optical modulator100, an optical system such as a lens and the optical fiber that inputand output an optical signal, a control unit that controls operationssuch as optical modulation, and the like. The optical module causes alight beam emitted by a light source to enter the optical modulator 100,optically modulates an electrical signal transmitted by the opticalmodulator 100, and outputs the optically modulated electrical signal toa transmission path such as the optical fiber. The optical module mayfurther include a configuration on the reception side that receives anoptical signal in the optical transmission path (such as, for example, areceiving unit and an optical demodulating unit). When the opticalmodule includes the transmission side and the reception side, a controlunit may further be provided therein that collectively controls thetransmitting unit and the receiving unit.

The LN chip 102 is provided with an optical waveguide 111 (indicated bya dotted line in FIG. 1) that guides the light beam entering thereintofrom the light source, from an entrance end 102 a to an exit end 102 bover a predetermined length, and the optical signal optically modulatedin the optical waveguide 111 portion is output from the other end 102 btoward the transmission path. The entrance end 102 a and the exit end102 b are each obliquely cut and are each connected to the optical fiber105 or the like. For connection of a lens system or the like, reflectionreduction treatment such as AR coating is applied to the entrance end102 a and the exit end 102 b portions of the optical waveguide 111 toreduce the reflectance of the light beam entering thereinto or exitingtherefrom.

In the example of the configuration of FIG. 1, the optical waveguide 111is branched into two along the length direction (the direction of travelof the light beam) and linear-shaped signal electrodes 121 are providedalong the length direction of each of the branched optical waveguides111. Ground electrodes (grounded electrodes) 122 each having a largerwidth than that of the signal electrode 121 are provided in the portionson both sides of the signal electrodes 121. The portion of the signalelectrodes 121 (and the ground electrodes 122) provided along theoptical waveguide 111 functions as an interacting unit for the opticalmodulation, data of an electrical signal to be transmitted is output tothe signal electrode 121, and the interacting unit of the opticalwaveguide 111 thereby converts the electrical signal into an opticalsignal (optical modulation).

In the example of the configuration of FIG. 1, the relay substrate 103is provided on one end (an electrode connection portion A) of the signalelectrode 121 (and the ground electrode 122) of the LN chip 102.Although not depicted in FIG. 1, a terminating resistor is provided onthe other end of the signal electrode 121 (and the ground electrode 122)of the LN chip 102.

The relay substrate 103 is arranged facing the signal electrode 121 andthe ground electrode 122 portions of the LN chip 102. On the relaysubstrate 103, signal electrodes 131 connected to the signal electrodes121 of the LN chip 102 and the ground electrodes 132 connected to theground electrodes 122 of the LN chip 102 are provided. The end face ofthe LN chip 102 and the end face of the relay substrate 103 facing eachother are each formed at the cutting step, and the LN chip 102 and therelay substrate 103 are arranged having respective end faces adhered toeach other or having a predetermined gap therebetween.

The signal electrode 121 of the LN chip 102 and the signal electrode 131of the relay substrate 103 are electrically connected to each otherthrough a wire 140 set by wire bonding. The ground electrode 122 of theLN chip 102 is also electrically connected to the ground electrode 132of the relay substrate 103 through wires 140. A plug connector (aconnector) 142 is provided using solder or the like in a portion (thepackage 101 portion) on the side opposite to that of the electrodeconnection portion A of the relay substrate 103, and an electricalsignal (data) is input through the connector 142 such as a coaxial cableor the like connected to the signal electrode 131, and the groundelectrode 132 of the relay substrate 103.

Inside the package 101 of the optical modulator 100, components areadditionally provided on the lower face of the LN chip 102 such as atemperature adjusting structure that includes a heat leveling board, aPeltier device, and the like; a temperature detector; a photo-receiver(PD) that detects the power of an optical signal; and the like. Thecontrol unit executes various types of control for inputting of anelectrical signal (data), oscillating light beam of the light source,and the optical modulator (the bias current and the like). In addition,the control unit executes control for a constant temperature bycontrolling the temperature adjusting structure based on the temperaturedetected by the temperature detector, and the like.

FIG. 2 is an enlarged view of the electrode connection portion of theoptical modulator chip and the relay substrate depicted in FIG. 1. FIG.2 is an enlarged view of the electrode connection portion A of FIG. 1.The signal electrode 121 at the one end of the electrode of the LN chip102 is formed to have a width in the electrode connection portion A thatis large relative to the width in the interacting unit and includes aconverting portion (a tapered portion) 121 a having a large width asdepicted. The ground electrodes 122 provided in the portions on bothsides of the signal electrode 121 correspond to the tapered portions 121a of the signal electrodes 121, and tapered portions 122 a are formed tomaintain constant property impedance (for example, 50Ω).

The tapered portions 121 a of the signal electrodes 121 are each formedto have a width gradually becoming larger toward the end of the LN chip102 (the one end of the electrode), and the tapered portions 122 a ofthe ground electrodes 122 are each formed to have a width graduallybecoming smaller toward the end of the LN chip 102 (the one end of theelectrode).

The end of the signal electrode 121 of the LN chip 102 and the signalelectrode 131 of the relay substrate 103 are electrically connected toeach other through the wire 140 set by wire bonding. The groundelectrode 122 of the LN chip 102 is also electrically connected to theground electrode 132 of the relay substrate 103 through the wires 140.

In the example of FIG. 1, the end of the signal electrode 121 of the LNchip 102 and the signal electrode 131 of the relay substrate 103 areconnected to each other by one wire 140, and the end of the groundelectrode 122 of the LN chip 102 and the ground electrode 132 of therelay substrate 103 are connected to each other by plural (two or three)wires 140. The number of wires 140 may arbitrarily be variedcorresponding to the size of the electrode connection portion A.

High frequency characteristics may be obtained by reducing as much aspossible the wire length of each of the wires 140. To do this, it iseffective to reduce as much as possible a gap d between (to bring closeto each other) the end of the signal electrode 121 of the LN chip 102and the end of the signal electrode 131 of the relay substrate 103. Inthis case, it is also effective to reduce as much as possible the samegap d between (to position close to each other) the end of the groundelectrode 122 of the LN chip 102 and the end of the ground electrode 132of the relay substrate 103.

FIGS. 3A and 3B are explanatory plan diagrams of chippings generated inthe existing optical modulator chip. FIG. 3A depicts the LN chip 102portion and does not depict the optical waveguide 111 for convenience.FIG. 3B is an enlarged view of the electrode connection portion A of theLN chip 102 and does not depict the wires 140.

When the LN chip 102 is cut out in the fabrication process, chippings301 are generated at a certain probability in cut surfaces 102 c and 102d along the length direction of the LN chip 102. The chippings 301 aregenerated having various lengths L1 and various depths D1 in the sidefaces of the LN chip 102 and, when the depths D1 are large, thechippings extend over the electrode portions to cause problems such aspeeling of the electrodes.

At present, the generation of such chippings 301 cannot be prevented.Therefore, the end 121 b of the signal electrode 121, and 122 b areformed at positions to each have a gap (spacing) d equal to or largerthan the depth D1 (several ten μm) to be the maximum in the ordinaryfabrication process to the cut surface 102 c of the LN chip 102 suchthat the chippings 301 do not extend over the electrodes even when thechippings 301 are generated. A width L0 of the end 121 b of the signalelectrode 121 is, for example, 100 μm.

Due to this, in the electrode connection part A, the LN chip 102 and therelay substrate 103 are separated from each other by the gap d equal toor larger than the depth D1 of the chipping 301. Although it iseffective to reduce the distance as much as possible between theelectrodes 131 and 132 of the relay substrate 103 and the electrodes 121and 122 of the LN chip to obtain excellent high frequencycharacteristics, when the end face of the relay substrate 103 is causedto adhere to the end face (the cut surface 102 c) of the LN chip 102,the electrodes are caused to be separated from each other by a distanceequal to or larger than the gap d consequent to providing the gap d andfrequency characteristics (high frequency characteristics) thereforecannot be improved. In addition, when the spacing is present between theLN chip 102 and the relay substrate 103, the electrodes are furtherseparated from each other and high frequency characteristics thereforedegrade.

The inventors focused their attention on the fact that the wire lengthof each of the wires 140 could be reduced and frequency characteristicscould be improved by further reducing the gap (the spacing) of theelectrode connection portion A (bringing close to the end face of the LNchip 102). Embodiments of the present invention will be described below.

FIGS. 4A and 4B are plan diagrams of an optical modulator chip accordingto a first embodiment. FIG. 4A depicts the LN chip 102 overall and FIG.4B is an enlarged view of the electrode connection portion A. In thefirst embodiment, the position of the end 121 b of the signal electrode121 is brought close to that of the end (the cut surface 102 c) of theLN chip such that the gap between the LN chip 102 and the relaysubstrate 103 is reduced only for the electrode connection portion A ofthe LN chip 102 that faces the relay substrate 103.

The gap is set to be “d” between the end 122 b of the ground electrode122 and the end (the cut surface 102 c) of the LN chip 102. A gap(spacing) “d1” between the end 121 b of the signal electrode 121 and theend (the cut surface 102 c) of the LN chip 102 is set to be smaller thanthe gap d of the ground electrode 122.

The pad (the end 121 b) of the signal electrode 121 has the width L0(for example, 100 μm) and the LN chip 102 has, for example, a length ofabout 40 mm to about 50 mm and a width of about 1 mm. The width L0 ofthe pad (the end 121 b) of the signal electrode 121 is, for example, 100μm and the remaining portion is mostly the ground electrode 122.

Observing along the length direction (the cut surface 102 c) of theelongated LN chip 102, the ratio that the widths L0 of the ends 121 b ofthe signal electrodes 121 occupy of the electrode connection portion Ais several percent of the length of the LN chip 102 depending on thenumber of the signal electrodes, and this ratio is low.

Even when chippings 301 (see FIGS. 3A and 3B) are generated at the stepof cutting the LN chip 102, the probability is low that the chippings301 are generated at the end 121 b of the signal electrode 121 of theelectrode connection portion A. The chippings 301 have a predeterminedlength L1 and are not generated in the overall area along the lengthdirection of the LN chip 102 but rather are generated at randompositions.

Consequently, even when only the end 121 b of the signal electrode 121is caused to protrude in the direction of the end (the cut surface 102c) of the LN chip 102, the probability that the chippings 301 will begenerated in the electrode connection portion A may be decreased to beextremely low. Even when the electrodes are formed with a small intervalequal to or smaller than the gap d with respect to the end 102 c of theLN chip 102 and the LN chip 102 is similarly cut based on the existinggap d, the rate of generation of the chippings 301 at the end 121 bportion of the signal electrode 121 is close to zero.

FIG. 5 is an enlarged view of FIG. 4A and is a diagram depicting a stateof the wire connection. FIG. 5 depicts a state in which the end face(the cut surface 102 c) along the length direction of the LN chip 102and the end 103 c of the relay substrate 103 are brought close to eachother. The end 131 b of the signal electrode 131 of the relay substrate103 and the end 132 b of the ground electrode 132 are each providedinward by a predetermined width from the end face (the cut surface) 103c.

As described, the end 121 b of the signal electrode 121 of the LN chip102 may be brought close to the signal electrode 131 of the relaysubstrate 103 whereby connection may be established using a wire 140 ahaving a short wire length between the end 121 b of the signal electrode121 of the LN chip 102 and the signal electrode 131 of the relaysubstrate 103, and the frequency property may be improved.

In the example depicted in FIG. 5, the LN chip 102 and the relaysubstrate 103 are arranged adhered to each other. Without limitationhereto, high frequency characteristics may also be improved even in thecase of a structure in which the distance between the end 102 c of theLN chip 102 and the end 103 c of the relay substrate 103 is several μmto several ten μm taking into consideration the difference in the linearexpansion coefficient between the respective materials.

FIGS. 6A, 6B, 6C, 6D, 6E, 6F are cross-sectional diagrams of an exampleof fabrication steps of the optical modulator chip according to thefirst embodiment. FIGS. 6A, 6B, 6C, 6D, 6E, 6F depict enlarged views ofcross-sections of a portion of the LN chip 102 in the wafer process andthe dicing.

As depicted in FIG. 6A, a film is first formed by vapor deposition oftitanium (Ti) 601 on an LN substrate (wafer) 102A. As depicted in FIG.6B, patterning by photolithography or the like is executed thereafter sothat the portion having the optical waveguide 111 formed thereinremains. As depicted in FIG. 6C, Ti is thereafter diffused to fabricatethe optical waveguide 111 in the LN substrate (the wafer) 102A.

As depicted in FIG. 6D, a buffer layer 602 and a silicon (Si) coatingfilm 603 are formed thereafter each as a film on the LN substrate (thewafer) 102A by electron beam (EB) vapor deposition or sputtering.

As depicted in FIG. 6E, the electrodes (the signal electrode 121 and theground electrode 122) are formed by executing gold (Au)-plating. Theprocess steps up to the formation of the electrodes are executed foreach one wafer as a unit.

As depicted in FIG. 6F, the LN substrate (the wafer) 102A is thereaftercut using a dicing saw or the like into chips each as the LN chip 102.At this step, as described, the chippings 301 and the like are generatedat the end face (such as 102 c) of the LN chip 102.

Consequently, for the end face (the cut surface) 102 c of the LN chip102, the cutting is executed at a position away from the electrode bythe predetermined gap d (several ten μm). FIG. 6F depicts the positionsfor cutting for the ground electrodes 122. The positions for cutting inthe first embodiment (FIGS. 4A and 4B) are the same positions as thepositions for existing (currently executed) cutting (FIGS. 3A and 3B).

In the first embodiment, the formation of the electrodes (the signalelectrode 121 and the ground electrode 122) are coped with by the maskdesign and the cutting is therefore executed after the completion of theformation of the electrodes (the signal electrode 121 and the groundelectrode 122) without any change relative to the existing electrodeformation process. At the step of dicing itself, the cutting is alsoexecuted using a dicing saw or the like in a process similar to that ofthe existing technique.

In this manner, all the steps relating to the formation of the LN chip102 can be executed using the cutting step based on the existingtechniques and the gap d similar to the existing one as they are, andthe LN chip 102 of the first embodiment can therefore be easily formed.In this case, any precision improvement is also not necessary for theprecision of the positions for the cutting and the like.

FIG. 7A is a chart of characteristics of the optical modulator chipaccording to the first embodiment. FIGS. 7B and 7C are diagrams ofexamples of LN chips. FIG. 7A depicts the result of simulation of S11(the reflection property). The vertical therein represents S11 (thereflection property [dB]) and the horizontal therein represents thefrequency [GHz].

FIG. 7B depicts an example of a current ordinary LN chip, i.e., a casein which the gap is d between the end 102 c of the LN chip 102 and theends of the electrodes (the end 121 b of the signal electrode 121 andthe end 122 b of the ground electrode 122) depicted in FIGS. 3A and 3Band the like.

FIG. 7C depicts an example of the LN chip 102 according to the firstembodiment, i.e., a case in which the gap is d between the end 102 c ofthe LN chip 102 and the end 122 b of the ground electrode 122, and thegap is d1 between the end 102 c of the LN chip 102 and the end 121 b ofthe signal electrode 121 depicted in FIGS. 4A, 4B and the like.

As depicted in FIG. 7A, according to the first embodiment, thereflection property is improved compared to that of the existingconfiguration in the overall region of the high frequency band. This isbecause the interval is reduced between the signal electrodes 121 and131 between the relay substrate 103 and the LN chip 102, and the wirelength of each of the wires 140 a that are provided between the signalelectrodes 121 and 131 is therefore short.

According to the first embodiment, the length of each of the wires thatconnect the signal electrodes between the LN chip and the relaysubstrate may be shortened by forming among the electrodes on the LNchip, the signal electrodes connected to the relay substrate so that theends are positioned close to the end face of the LN chip. The frequencyproperty may thereby be improved. Thus, an LN chip may be provided thathas improved frequency characteristics by a simple step withoutdegrading the fabrication yield of the LN chip because the probabilityis extremely low that chipping generated at the step of cutting the LNchip will be generated at the portions of the signal electrodes eachhaving a small width.

FIGS. 8A and 8B are plan diagrams of an optical modulator chip accordingto a second embodiment. FIG. 8A depicts the LN chip 102 overall and FIG.8B is an enlarged view of the electrode connection portion A. In thesecond embodiment, concerning the electrode connection portion A of theLN chip 102 that faces the relay substrate 103, similar to the firstembodiment, the end 121 b of the signal electrode 121 is brought closeto the end (the cut surface 102 c) of the LN chip 102 such that the gapbetween the relay substrate 103 and the LN chip 102 is reduced.

In the second embodiment, some of the ends 122 b of the groundelectrodes 122 are brought close to the end (the cut surface 102 c) ofthe LN chip 102.

A portion of the ground electrode 122 (a predetermined length L3portion) positioned in the electrode connection portion A and adjacentto the signal electrode 121, includes an end (a first tip) 122 c that isbrought close to the end (the cut surface 102 c) of the LN chip 102. Thegap (the spacing) is d1 between the end (the first tip) 122 c and theend (the cut surface 102 c) of the LN chip 102.

A portion of the ground electrode 122 of the LN chip 102 (the portionother than the predetermined length L3 of the first tip 122 c) away fromthe signal electrode 121, includes the end (a second tip) 122 b of thegap d (d>d1) between the portion and the end (the cut surface 102 c) ofthe LN chip 102. The second tip 122 b has the gap d similar to that ofthe first embodiment to the end (the cut surface 102 c) of the LN chip102. In the planar view, the first end 122 c is formed including a stepthat protrudes more toward the end (the cut surface 102 c) of the LNchip 102 than the second tip 122 b.

As described, the end 121 b of the signal electrode 121 positioned inthe electrode connection portion A and the portion of the end 122 c ofthe ground electrode 122 (the portion adjacent to the signal electrode121) are brought close to the end (The cut surface 102 c) of the LN chip102 to have the gap d1 therebetween. In this case, between the LN chip102 and the relay substrate 103, in addition to the signal electrode121, the ground electrode 122 as well as may be connected by the pluralshort wires 140. Configuration may be such that the gap d1 retained bythe end 121 b of the signal electrode 121 and that retained by the end122 c of the ground electrode 122 are not be equal to each other.

In FIGS. 8A and 8B, the electrode connection portion A is depicted beingenlarged (enlarged in length) compared to the length of the overall LNchip 102 and the length of the electrode connection portion A along thelength direction is several hundred μm. Consequently, even when the end122 c of the ground electrode 122 is brought close to the end (the cutsurface 102 c) of the LN chip 102 with the gap d1 present therebetween,the rate of generation of the chippings 301 at the end 122 c of theground electrode 122 when the cutting is executed is still low.

As described, according to the second embodiment, concerning theelectrode connection portion, not only the signal electrode but also theground electrode may be connected each using a wire having a shortlength. Production of ripples and the like, and fine adjustment(matching) of the property impedance may thereby be easily executed andthe frequency band may further be improved.

FIGS. 9A and 9B are plan diagrams of the optical modulator chipaccording to a third embodiment. FIG. 9A depicts the overall LN chip 102and FIG. 9B is an enlarged view of the electrode connection portion A.In the third embodiment, for the electrode connection portion A of theLN chip 102, similar to the above embodiments, the end 121 b of thesignal electrode 121 is brought close to the end (the cut surface 102 c)of the LN chip 102 such that the gap is reduced between the relaysubstrate 103 and the LN chip 102.

In the third embodiment, the end 122 c is provided that is formed bybringing a portion of the ground electrode 122 to be close to the end(the cut surface 102 c) of the LN chip 102 with the gap d1 presenttherebetween.

As depicted in FIG. 9B, the gap is set to be d between the end 122 c ofthe ground electrode 122 and the end (the cut surface 102 c) of the LNchip 102. Of the ground electrode 122, the end 122 c having apredetermined length L3 brought close to the end (the cut surface 102 c)of the LN chip 102 is provided in the portion positioned in theelectrode connection portion A. The gap (the spacing) is set to be d1between the end 122 c and the end (the cut surface 102 c) of the LN chip102.

Of the portion of the ground electrode 122 positioned in the electrodeconnection portion A, a broad portion (an inclining portion) 122 d forwhich the interval of the gap continuously varies from that at theposition of the end 122 c (the gap d1) to that at the position of thegap d is provided in an area other than the end 122 c.

The broad portion 122 d of the ground electrode 122 has a shape whoseportion in proximity to the signal electrode 121 portion (the end 122 c)is positioned closest to the end (the cut surface 102 c) of the LN chip102 to have a small gap. The shape of the ground electrode 122 at aposition more distant from the signal electrode 121 along the lengthdirection of the LN chip 102 is such that the ground electrode 122 ismore distant from the end (the cut surface 102 c) of the LN chip 102 dueto the broad portion 122 d (the gap becomes larger).

As described, the end 121 b of the signal electrode 121 positioned inthe electrode connection portion A and the portion of the end 122 c ofthe ground electrode 122 (the point in proximity to the signal electrode121) are brought close to the end (the cut surface 102 c) of the LN chip102. In this case, between the LN chip 102 and the relay substrate 103,in addition to the signal electrode 121 the ground electrode 122 mayalso be connected by the plural short wires 140.

In the broad portion 122 d, the gap continuously varies therein howeverthe shape thereof is not limited hereto and the broad portion 122 d maybe configured to have plural different gaps and the gaps may be variedstepwise.

Consequently, in the third embodiment, concerning the electrodeconnection portion, not only the signal electrode but also the groundelectrode may also be connected using wires each having a short length.The occurrence of ripples and the like, and fine adjustment (matching)of the property impedance may thereby be easily performed and thefrequency band may further be improved.

In addition, in the third embodiment, the occurrence of ripples and thelike, and fine adjustment of the property impedance (matching) may beexecuted more easily than those of the second embodiment and thefrequency band may be improved further by providing the broad portion inthe ground electrode in the electrode connection portion.

FIGS. 10A and 10B are plan diagrams of the optical modulator chipaccording to a fourth embodiment. FIG. 10A depicts the LN chip 102overall and FIG. 10B is an enlarged view of the electrode connectionportion A.

In the fourth embodiment, the structure of the electrode connectionportion (a first electrode connection portion) A on the input side forthe electrical signal to the electrode is also applied to the outputside of the electrode, i.e., the portion (a second electrode connectionportion B) that is connected to the terminating resistor and the like.The electrode connection portions A and B are both provided on the sameend (the cut surface 102 c) of the LN chip 102.

As described, for the electrode connection portion A, any one of thevarious aspects of the first to the third embodiments (FIGS. 4A and 4B,FIGS. 8A and 8B, and FIGS. 9A and 9B) may be employed. For the electrodeconnection portion B, any one of the various aspects of the first to thethird embodiments (FIGS. 4A and 4B, FIGS. 8A and 8B, and FIGS. 9A and9B) may also be employed. FIGS. 10A and 10B depict an example where thestructure described in the first embodiment (FIGS. 4A and 4B) is appliedto each of the electrode connection portions A and B.

According to the fourth embodiment, the effects described in the aboveembodiments may be achieved and, for both the input side and the outputside for the electrical signal to/from the LN chip, the high frequencycharacteristics and ripples of the terminating portion may be reducedfurther by bringing the end of each of the signal electrode and theground electrode to be in proximity to the end face (the cut surface) ofthe LN chip.

FIGS. 11A, 11B, and 11C are plan diagrams of an optical modulator chipaccording to a fifth embodiment. FIG. 11A depicts the LN chip 102overall; FIG. 11B is an enlarged view of the electrode connectionportion A; and FIG. 11C is an enlarged view of the electrode connectionportion B.

In the fifth embodiment, application is made to an example where theoutput side of the electrodes, i.e., the portion connected to theterminating resistor and the like (the electrode connection portion B)is formed on the opposite face of the LN chip 102 to the electrodeconnection portion A on the input side for the electrical signals to theelectrodes. The electrode connection portions A and B are provided onends (the cut surfaces 102 c and 102 d) of the faces opposite to eachother of the LN chip 102.

Similar to the fourth embodiment, for the electrode connection portionsA and B, any one of the various aspects of the first to the thirdembodiments (FIGS. 4A and 4B, FIGS. 8A and 8B, and FIGS. 9A and 9B) maybe employed. FIGS. 11A, 11B, and 11C depict an example where thestructure described in the first embodiment (FIGS. 4A and 4B) is appliedto each of the electrode connection portions A and B.

According to the fifth embodiment, the effects described in the aboveembodiments may be achieved and, the same actions and effects may alsobe achieved even when the input side and the output side for electricalsignals into/from the LN chip are formed on the faces opposite to eachother of the LN chip. The high frequency characteristics and the ripplesin the terminating portion may be reduced further by bringing the endsof the electrodes (the signal electrode and the ground electrode)provided on the opposite faces of the LN chip 102 to be in proximity tothe end faces (the cut surfaces) of the LN chip.

According to the embodiments, the length of each of the wires connectingthe LN chip and the signal electrodes of the relay substrate to eachother may be reduced by forming among the electrodes on the LN chip, theend of at least the signal electrode positioned in the electrodeconnection portion that connects to the relay substrate, to bepositioned close to the end face of the LN chip. The end of the groundelectrode may be formed so that the position of the end is brought closeto the end face of the LN chip. The portion at the position adjacent tothe signal electrode of the end of the signal electrode may be formed insteps bringing the position thereof close to the end face of the LNchip. The portion at the position adjacent to the signal electrode ofthe end of the signal electrode may be formed bringing the positionclose to the end face of the LN chip and may be formed such that theposition is gradually more distant from the end face of the LN chip asthe position is more distant from the signal electrode.

Frequency characteristics may be improved by a simple structure byemploying the above configuration. An LN chip may be provided that hasfrequency characteristics improved by a simple step without degradingthe fabrication yield of the LN chip because the probability isextremely law that chipping generated at the step of cutting the LN chipwill be generated at the portion of the signal electrode having a smallwidth.

Although the modulator chip has been described taking an example of theLN (LiNbO₃) chip in the embodiments, the modulator chip is not limitedto the LN chip and the frequency characteristics may also be improvedeven when the chip is an InP or an Si chip. Although the number ofsignal electrodes and the number of branches of the optical waveguideare each set to be two, the numbers are not limited hereto and a pluralnumber (for example, four, eight, . . . ) may be employed.

To obtain wideband frequency characteristics, it is effective to reduceas much as possible the length of the wire connecting an electrode onthe chip to a relay substrate and, to do this, the electrode on the chipand an electrode on the relay substrate are brought close to each other.A conventional LN chip is fabricated using a semiconductor wafer processor the like and is finally formed as a chip at a step of cutting thesemiconductor wafer by dicing or the like while chippings (correspondingto burrs) each having a predetermined length are randomly generated oneach of the cut faces (on ends of the electrode portions) at apredetermined probability at the cutting step.

As to chipping, when chipping extends over an electrode (plated) portionon an end of the chip, a problem arises in that the plating is peeledoff from the portion and the like. The cutting is therefore executedusually causing the end of the electrode to be distant from the end ofthe LN chip by a distance equal to or larger than several ten μm suchthat no chipping extends over the electrode (the plating). Due to this,a gap is formed between the end of the LN chip and the LN electrode, andthe formed gap degrades high frequency characteristics to an extentcorresponding to the size of the gap.

According to an embodiment, an effect is achieved in that high frequencycharacteristics may be improved without being affected by chipping.

Although the invention has been described with respect to a specificembodiment for a complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art which fairly fall within the basic teaching hereinset forth.

What is claimed is:
 1. An optical modulator comprising: an opticalmodulator chip configured to optically modulate an optical signal usingan electrical signal input thereto; and a relay substrate configured torelay and couple the electrical signal to the optical modulator chip,wherein the optical modulator chip includes a signal electrode and aground electrode for the electrical signal, formed along a waveguide forthe optical signal, one end of the optical modulator chip is arranged toface the relay substrate, and an electrode connection portion isconfigured to couple the electrical signal to the relay substrate usinga wire is provided at the one end, the ground electrode has: a first tipthat is adjacent to the signal electrode; and a second tip that isdistant from the signal electrode, and a distance between a tip of oneend of the signal electrode positioned in the electrode connectionportion and the one end of the optical modulator chip, and a distancebetween the first tip of one end of the ground electrode and the one endof the optical modulator chip are both less than a distance between thesecond tip of the one end of the ground electrode and the one end of theoptical modulator chip.
 2. The optical modulator according to claim 1,comprising a second electrode connection portion in which other ends ofthe signal electrode and the ground electrode are provided to face theone end of the optical modulator chip, the other ends of the signalelectrode and the ground electrode being connected to a terminatingportion facing thereto, by wires at the one end of the optical modulatorchip.
 3. The optical modulator according to claim 1, comprising a secondelectrode connection portion in which other ends of the signal electrodeand the ground electrode are provided to face a second end of theoptical modulator chip, the other ends of the signal electrode and theground electrode being connected to a terminating portion facing theretousing wires at the second end of the optical modulator chip.
 4. Theoptical modulator according to claim 1, wherein a material of asubstrate of the optical modulator chip is a ferroelectric.
 5. Theoptical modulator according to claim 1, wherein an end of the opticalmodulator chip and the relay substrate are adhered to each other or arearranged having a predetermined interval therebetween.
 6. An opticalmodule comprising: the optical modulator according to claim 1; a sourceof a light beam to be input into the optical waveguide; an opticalsystem for the optical signal; and a controller configured to control anoperation of the optical modulator.
 7. An optical modulator comprising:an optical modulator chip configured to optically modulate an opticalsignal using an electrical signal input thereto; and a relay substrateconfigured to relay and couple the electrical signal to the opticalmodulator chip, wherein the optical modulator chip includes a signalelectrode and a ground electrode for the electrical signal, formed alonga waveguide for the optical signal, one end of the optical modulatorchip is arranged to face the relay substrate, and an electrodeconnection portion configured to couple the electrical signal to therelay substrate using a wire is provided at the one end, the groundelectrode positioned in the electrode connection portion has: a firsttip that is adjacent to the signal electrode; and a broad portion havinga distance to the one end of the optical modulator chip becoming largeras the broad portion becomes more distant from the signal electrode, anda distance between a tip of one end of the signal electrode positionedin the electrode connection portion and the one end of the opticalmodulator chip, and a distance between the first tip of one end of theground electrode and the one end of the optical modulator chip are bothless than a distance between the broad portion of the ground electrodeand the one end of the optical modulator chip.
 8. The optical modulatoraccording to claim 7, comprising a second electrode connection portionin which other ends of the signal electrode and the ground electrode areprovided to face the one end of the optical modulator chip, the otherends of the signal electrode and the ground electrode being connected toa terminating portion facing thereto, by wires at the one end of theoptical modulator chip.
 9. The optical modulator according to claim 7,comprising a second electrode connection portion in which other ends ofthe signal electrode and the ground electrode are provided to face asecond end of the optical modulator chip, the other ends of the signalelectrode and the ground electrode being connected to a terminatingportion facing thereto using wires at the second end of the opticalmodulator chip.
 10. The optical modulator according to claim 7, whereina material of a substrate of the optical modulator chip is aferroelectric.
 11. The optical modulator according to claim 7, whereinan end of the optical modulator chip and the relay substrate are adheredto each other or are arranged having a predetermined intervaltherebetween.
 12. An optical module comprising: the optical modulatoraccording to claim 7; a source of a light beam to be input into theoptical waveguide; an optical system for the optical signal; and acontroller configured to control an operation of the optical modulator.